Théodore Biadala
--- a/composer/Metapackage/CoreRecommended/composer.json
+++ b/composer/Metapackage/CoreRecommended/composer.json
    "require": {
        "drupal/core": "9.3.x-dev",
        "asm89/stack-cors": "1.3.0",
+        "axy/backtrace": "1.0.7",
+        "axy/codecs-base64vlq": "1.0.1",
+        "axy/errors": "1.0.5",
+        "axy/sourcemap": "0.1.5",
--- a/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
    // grouping, optimizing and dumping, but change the strategy that is used to
    // determine when the aggregate should be rebuilt (e.g. mtime, HTTPS …).
    $map = $this->state->get('system.js_cache_files', []);
+
+    $generate_sourcemaps = \Drupal::config('system.performance')->get('js.sourcemaps');
--- a/composer.lock
+++ b/composer.lock
    "platform-overrides": {
        "php": "7.3.0"
    },
-    "plugin-api-version": "2.1.0"
+    "plugin-api-version": "2.0.0"
--- a/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+                'sections' => [],
+              ];
              foreach ($js_group['items'] as $js_asset) {
                // Optimize this JS file, but only if it's not yet minified.
                if (isset($js_asset['minified']) && $js_asset['minified']) {
-                  $data .= file_get_contents($js_asset['data']);
+                  $file_content = file_get_contents($js_asset['data']);
                }
                else {
-                  $data .= $this->optimizer->optimize($js_asset);
+                  $file_content = $this->optimizer->optimize($js_asset);
+                }
+                if ($generate_sourcemaps) {
+                  $js_map = FALSE;
+                  // pick up map files automatically, even if they're not declared
+                  // in the source (looking at you jquery)
--- a/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+                    $js_map = json_decode(file_get_contents($js_asset['data'] . '.map'));
+                  }
+                  elseif (preg_match('~//[#@]\s(?:source(?:Mapping)?URL)=\s*(\S+)\s*~', $file_content, $matches)) {
+                    if (str_contains($matches[1], 'data:application/json;')) {
+                      $base64 = str_replace('data:application/json;charset=utf-8;base64,', '', $matches[1]);
+                      $js_map = json_decode(base64_decode($base64));
+                    }
+                    else {
+                      $map_file = pathinfo($js_asset['data'], PATHINFO_DIRNAME) . '/' . $matches[1];
+                      if (file_exists($map_file)) {
+                        $js_map = json_decode(file_get_contents($map_file));
+                      }
+                    }
+                  }
+                  $sourcesRoot = \Drupal::service('file_url_generator')
+                    ->generateAbsoluteString(pathinfo($js_asset['data'], PATHINFO_DIRNAME));
--- a/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/JsCollectionOptimizer.php
+                  $sourcesRoot = \Drupal::service('file_url_generator')
+                    ->generateAbsoluteString(pathinfo($js_asset['data'], PATHINFO_DIRNAME));
+                  if ($js_map) {
+                    // Make sure the source shows up in the right place.
+                    $js_map->sourceRoot = $sourcesRoot;
+                    $section = [
+                      'offset' => [
+                        'line' => substr_count($data, "\n"),
+                        'column' => 0,
+                      ],
+                      'map' => $js_map,
+                    ];
+                  }
+                  // if there are not sourcemap create one to avoid problems when
+                  // setting breakpoints. This happens when core js has not been
+                  // generated with yarn build:js-dev